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In this work, we identify the type of calcium arsenates found in sediment samples from an aquifer located in Matehuala, San Luis Potosí, México. Sediments in contact with levels up to 158 mg/L of arsenic in neutral pH water were studied by X-ray diffraction, scanning electron microscopy coupled to energy dispersive X-ray analyses (SEM-EDS), and synchrotron based X-ray diffraction. Identification of these calcium arsenates by X-ray analysis has proved to be very difficult to achieve because the precipitates of interest are on the microscale and immerse in a matrix of calcite, gypsum, and quartz comprising nearly 100 % of the samples. Needle-like specimens composed of calcium, arsenic, and oxygen were, however, commonly observed in sediment samples during SEM-EDS analyses in backscattered mode. Synchrotron based X-ray analyses revealed some peaks that were compared with published data for guerinite, haindingerite, and pharmacolite suggesting that these were the calcium arsenates present in sediments, the calcium arsenates that control the solubility of arsenic in the contaminated aquifer in Matehuala, and the calcium arsenates that prevail in the long-term in the environment after cycles of dissolution and precipitation. The identification of these calcium arsenates is consistent with the environmental conditions prevailing at the study area and the SEM-EDS observations. However, its identification is not unequivocal as the comparison of experimental data collected in single crystal specimens against X-ray diffraction references collected in powders prevents a strictly proper identification of the specimens analyzed. In this way, scorodite was also identified by synchrotron based Xray analyses however its presence is inconsistent with the environmental conditions and the calcium arsenate associations found in this study. Scorodite identification was therefore considered tentative. A thorough examination, with additional and/or improved analytical techniques, should be undertaken to find an environmentally sound explanation for the diffraction peaks assigned to scorodite, which might be from a clay a mineral, probably with no arsenic. [ABSTRACT FROM AUTHOR]

An As-contaminated perched aquifer under an urban area affected by mining was studied over a year to determine the contamination source species and the mechanism of As mobilization. Results show that the dissolution of calcium arsenates in residues disposed on an inactive smelter has caused high levels of As pollution in the adjoining downgradient 6-km perched aquifer, reaching up to 158 mg/L of dissolved As, and releasing a total of ca. 7.5 tons of As in a year. Furthermore, free calcium ion availability was found to control As mobility in the aquifer through the diagenetic precipitation of calcium arsenates (Ca5H2(AsO4)4·cH2O) preventing further mobilization of As. Results shown here represent a model for understanding a highly underreported mechanism of retention of arsenate species likely to dominate in calcium-rich environments, such as those in calcareous sediments and soils, where the commonly reported mechanism of adsorption to iron(III) oxyhydroxides is not the dominant process. [Copyright &y& Elsevier]

Abstract: Long-term stability of arsenic residues is investigated by determining arsenic phases remaining in gold mining residues after two decades of impoundment. The residues, generated by arsenic coprecipitation with iron and lime, were disposed of in-lined sites for 9–16 years (pit C) and 16–23 years (pits A and B). Arsenic is present in the residues as As(V) species, predominantly in the form of amorphous iron arsenate (55–75% Astotal, pits A and B; 55–70% Astotal, pit C) and sorbed onto amorphous iron-oxyhydroxides (20–33% Astotal, pits A and B; 22–37% Astotal, pit C). The presence of minor Ca–arsenate phases (undefined composition) and Al–arsenate coprecipitates is also indicated. The passive enrichment of iron in pits A and B, and the relative low concentration of calcium, sulfur and arsenic if compared to those of pit C, suggest that a soluble Ca–arsenate phase (e.g. CaHAsO4.H2O), a fraction of gypsum and As(III) were dissolved along 16–23 years of residue disposal. The presence of As(V) only and excess iron demonstrates the importance of the oxidation state and high Fe/As ratio on long-term stability of arsenic residues. [ABSTRACT FROM AUTHOR]

The solubility and stability of calcium arsenates at 25 ∘C was determined by both precipitation and dissolution experiments. Ca3(AsO4)2⋅ 3H2O(c), Ca3(AsO4)2⋅ 21/4H2O(c), Ca5(AsO4)3(OH)(c) and Ca4(OH)2(AsO4)2⋅ 4H2O(c) were identified in our experiment over a wide range of pH and for Ca/As molar ratios between 1.25 and 4.0. The solids precipitated at pH = 3 ∼ 7 and Ca/As = 1.5 were phase-pure and well-crystallized Ca3(AsO4)2⋅ xH2O(c) and had relatively larger grain size than those formed at pH > 7. Based on the analytical results and using the computer program PHREEQC, the solubility products for Ca3(AsO4)2⋅ 3H2O(c), Ca3(AsO4)2⋅ 21/4H2O(c), Ca5(AsO4)3(OH)(c) and Ca4(OH)2(AsO4)2⋅ 4H2O(c) were calculated as K sp of 10− 21.14(10− 20.01 ∼ 10− 22.02), 10− 21.40(10− 20.08 ∼ 10− 21.98), 10− 40.12(10− 37.53 ∼ 10− 42.72) and 10− 27.49(10− 26.10 ∼ 10− 28.91), respectively. Correspondingly, the free energies of forming (Δ G f ) of these calcium arsenates were calculated to be −3787.87 kJ/mol, −3611.50 kJ/mol, −5096.47 kJ/mol and −4928.86 kJ/mol. [ABSTRACT FROM AUTHOR] o ) of these calcium arsenates were calculated to be −3787.87 kJ/mol, −3611.50 kJ/mol, −5096.47 kJ/mol and −4928.86 kJ/mol. [ABSTRACT FROM AUTHOR]

Calcium arsenates such as pharmacolite (CaHAsO 4 ·2H 2 O), haidingerite (CaHAsO 4 ·H 2 O), and weilite (CaHAsO 4 ) are important sinks for arsenic in mine tailings as well as other natural and contaminated sites and are useful for reducing the mobility and bioavailability of this toxic metalloid in the environment. However, calcium arsenates usually occur in trace amounts dominated by other phases, making their detection, identification, and quantification challenging. In this contribution, pharmacolite, haidingerite, and weilite are shown to exhibit subtle but distinct postedge differences in As K-edge X-ray absorption near-edge structure (XANES) spectra and feature characteristic [AsO 3 ] 2- , [AsO 4 ] 2- , and [AsO 4 ] 4- radicals, all derived from the diamagnetic [HAsO 4 ] 2- precursor during γ-ray irradiation, in electron paramagnetic resonance (EPR) spectra. In particular, the 75 As (nuclear spin I = 3/2 and natural isotope abundance = 100%) hyperfine coupling constants of the [AsO 3 ] 2- radicals in pharmacolite and haidingerite as well as other minerals (e.g., calcite and gypsum) are clearly distinct, allowing the unambiguous identification of calcium arsenates by the EPR technique readily at ∼0.1 wt %. Similarly, linear combination fittings of As K-edge XANES spectra demonstrate that pharmacolite and haidingerite at ∼0.1 wt % each in gypsum-rich mixtures can be detected and quantified as well. Therefore, a combination of the EPR and XANES techniques is a powerful approach for the highly sensitive characterization of calcium arsenates in the quest for the safe management and remediation of arsenic contamination. This work demonstrates the highly sensitive characterization of calcium arsenates by integrated electron paramagnetic resonance and synchrotron X-ray absorption spectroscopy.

The underground spaces of the former mines in Jáchymov, Czech Republic and Rotgülden, Austria house diverse assemblages of secondary minerals, mostly arsenates. The formation conditions and processes involved with the secondary minerals were deciphered using field observations, sampling of both solids and co-existing aqueous solutions, calorimetric measurements on Ca Mg arsenates, forward thermodynamic modeling, and X-ray absorption spectroscopy. In Jáchymov, the sampled solutions have pH values from 0.3 up to 7.4 and are moderately to strongly oxidized. The acidic solutions precipitate arsenolite and kaatialaite, and originate by the weathering of native arsenic and sulfides. When these solutions encounter hydrothermal carbonates they are slowly neutralized and precipitate the Ca Mg arsenates picropharmacolite, pharmacolite, haidingerite, brassite, and rösslerite. Measured enthalpies of formation and standard entropies were combined to Gibbs free energies of formation (all in kJ/mol) for picropharmacolite [Ca 4 Mg(AsO 3 OH) 2 (AsO 4) 2 ·11H 2 O, −8139.2 ± 12.4], pharmacolite [Ca(AsO 3 OH)·2H 2 O, −1762.0 ± 3.1], haidingerite [Ca(AsO 3 OH)·H 2 O, −1522.8 ± 3.1], rösslerite [Mg(AsO 3 OH)·7H 2 O, −2849.7 ± 2.5], and brassite [Mg(AsO 3 OH)·4H 2 O, −2139.9 ± 2.7]. Forward thermodynamic modeling and the calculation of temperature-relative humidity phase diagrams using these values agreed with field observations. In Rotgülden, secondary Ca Mg minerals are restricted to gypsum and hörnesite, and no aqueous solutions were sampled there. The Mg arsenates brassite and rösslerite are typical for systems which initially develop substantial acidity and metal load in the aqueous phase, and afterwards are neutralized, whereas the Mg arsenate hörnesite is typical for systems which maintain circumneutral or mildly basic pH, and are slowly enriched in As. Evaluation of the saturation indices in almost 900 chemical analyses of pristine water and acidic/neutral mine drainage showed that they are all undersaturated with respect to Ca Mg arsenates. Therefore, these minerals control the As solubility only in specific (carbonate-rich, Fe-poor, S-poor) mine drainage systems that are progressively neutralized. Unlabelled Image [ABSTRACT FROM AUTHOR]